The present invention relates to an apparatus for manufacturing bags made of plastic material, and in particular to an apparatus of the type comprising advancement means and welding, cutting, accumulation and extraction devices for manufacturing bags in thermoplastic material such as polyethylene.
As known, the manufacture of bags in plastic material employs machines which cause the advancement of a web of material defining a tubular sheath, and which weld it and cut it transversely at preset distances so as to form bags which are then gathered on needles or other systems and periodically unloaded and stacked. The web is caused to advance intermittently, by means of one or more pairs of rollers, between a pair of welding jaws, at least one of which is movable. The cutting operation is performed when the web is clamped by the welding jaws, generally by means of a blade carried by a belt along a path which intercepts said web or by means of guillotine blades or cutting combs.
In the case of a moving blade, the cutting operation is generally performed at high speed in order to require a very short time and thus avoid operating when the web is very hot, since by softening it would risk tearing, and to be able to remove the manufactured bag before the complete opening of the welding jaws, which in this manner allows advancement of the amount of web required for the manufacture of another bag.
Known machines produce an advancement of the web, for each working cycle, which corresponds to a rotation through 360 degrees of the primary shaft or camshaft. In particular, current welding and cutting devices operate at speeds in the order of 150 cycles per minute. This entails the constructin of very sturdy and sophisticated machines, since at such speeds wear and the possibility of breakdowns are obviously increased, adjustment is more complicated and maintenance is more demanding. Furthermore, conventional machines usually operate along a horizontal line, which increases the difficulty of discharging the manufactured bags at high speed. On the other hand slowing down the welding and cutting time compulsorily entails a respective decrease in the productivity of the machine.